Attic Ventilation

In our haste to build houses in the past twenty years, we have been careless at times with some very important details. Attic ventilation is one of them. A home that I inspected this weekend is a really good example. Upon entering the attic space I found a lovely even layer of cellulose insulation about 10 inches thick. It had been well installed. Often I see it in waves or looking like a mountain range. Cellulose is a good insulator and has become popular in the past ten years or so because it is cheaper than glass fibre. Its main use is attics where there is more room and where its tendency to pack down won’t be a problem as it would be in walls.

Poor ventilation in an attic space means that the air in the attic will change very slowly. Let’s say that we have a prolonged warm spell, like we have had a couple of times this winter. The air in the attic space becomes warm and more humid. If the temperature suddenly drops, just as it has, the humidity in the moisture condenses against the roof deck, especially on roofing nails or gang nails on trusses. It then freezes. If you were to enter the attic area at this point you would see a layer of frost on the underside of the roof deck or you might just see it on the nails. If you have seen this in your attic area, you don’t have enough ventilation. An ideal situation would be to have soffit vents with either gable vents, roof vents, or a ridge vent. Soffit vents being 50% of the area of the total ventilation and, oh yes, more is better!

The house I inspected had another complication. I could see a truss which appeared to be wet on top. Then my client noticed a black stain near some of the baffles on the underside of the roof deck. Upon testing with a moisture meter I found the truss was wet and got wetter as I moved down, toward the baffles. My immediate thought was a leak in the roof, but since water doesn’t flow uphill, that was impossible. The black stain turned out to be mildew and, again, it was wet, Over 19% humidity in wood means it is actively rotting. So the truss was rotting and mildew was forming on the underside of the roof deck. As we puzzled over this rather odd localized moisture problem, I crawled around the rest of the attic, finding that, although it was humid, it wasn’t actually wet. Crawling isn’t really that accurate for how I have to get around attic spaces. To avoid disturbing the insulation, I sort of walk on my side, my feet pushing on one web of the truss with my shoulder on another. Moving from one truss to another in this position is very tiring.

The best explanation I could come up with for my client was that since the bathroom was in the same general area as the signs of moisture in the attic, and the prevailing wind hit the house on that side, the moisture came from the bathroom fan. These fans are most commonly vented through the soffit. It should then be fitted with a proper boot to make sure this warm and very moist air goes outside. Sometimes this pipe is left to discharge inside the soffit as I believe it is here. In this case, the wind is pushing the warm moist air up into the attic, making this poorly ventilated attic even wetter.

I am constantly amazed at how much damage a little detail can cause. For the very small investment of hiring a home inspector and another small investment correcting the situation, my client will save himself thousands of dollars of repair work later on, when the roof would have collapsed.


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